1. Field of the Invention
The present invention relates to an analog-to-digital (A/D) converter, more particularly to a successive approximation type of analog-to-digital converter implemented in such as a large scale integration (LSI) circuit.
2. Description of the Background Art
Reference will first be made to FIG. 2 for describing a conventional analog-to-digital converter for better understanding an analog-to-digital converter according to the present invention. FIG. 2 is a schematic block diagram showing part of an LSI circuit including a conventional analog-to-digital converter. In FIG. 2, the LSI circuit 100 includes an analog-to-digital converter 10 adapted to convert analog signals A into corresponding digital signals D; and a logic circuit 20 for performing logical operations in response to the digital signals D to output signals OUT. The analog-to-digital converter 10 and logic circuit 20 are connected to a common supply voltage terminal 1 and common ground terminal 2 and are driven by a supply voltage provided from the supply voltage terminal 1. Signals are designated with reference numerals designating connections on which they are conveyed.
The analog-to-digital converter 10 is of a successive approximation type where the voltage of the analog signals A is sampled and held such that the capacitor 12 stores corresponding electric charges, which are in turn compared sequentially with a reference voltage, which is sequentially switched, to output the digital signal D. The input analog signals A are supplied through a sampling switch 11 to an internal node NI. Between the internal node NI and a ground node NG, the capacitor 12 for storing the input voltage is connected. Further, between a power supply node NV and a ground node NG, a resistance type potential divider 13 for generating reference voltages is connected. In the resistance type potential divider 13, any one of the reference voltages output from the resistance type potential divider 13 is selected by switches 14 to be supplied to a comparison controller 15. From the comparison controller 15, signals for controlling on/off action of the switch 11 and selection behavior of switches 14 are output to the switches 11 and 14, respectively.
The comparison controller 15 is adapted to compare the reference voltage selected by one of the switches 14 and the voltage stored in the capacitor 12 and appearing at the internal node NI, and, depending on the result of comparison, to operate the controls of the switches 14 so as to render the one of the switches 14 turn on and off to produce the digital signal D, which corresponds to the analog input signal A.
In the analog-to-digital converter 10, one end of the resistance type potential divider 13 and a terminal of the comparison controller 15 on the side of the supply voltage are connected in common to the power supply node NV, which is also connected to the supply voltage terminal 1 of the LSI circuit 100 via a power supply connection 30. In the analog-to-digital converter 10 also, the other end of the resistance type potential divider 13 and a terminal of the comparison controller 15 on the side of the ground are connected in common to the ground node NG, which is also connected to the ground terminal 2 of the LSI circuit 100 via a ground connection 32.
In the logic circuit 20, one terminal on the side of the supply voltage is connected to the supply voltage terminal 1 of the LSI circuit 100 via a power supply connection 34 and the other terminal on the side of the ground is connected to the ground terminal 2 via a ground connection 36.
Such an analog-to-digital converter for converting analog signals into corresponding digital signals is disclosed, for example, by U.S. Pat. No. 5,736,951 A1 to Kobatake, in which a number of comparators are used for comparing an analog input voltage to each of different reference voltages to output digital values, and, with each comparator, a supply voltage terminal and ground terminal are connected, respectively, in order to protect the comparator from an error caused by noise.
In the conventional analog-to-digital converter 10, the capacitor 12 is, however, subject to switching noise generated in the logic circuit 20. More specifically, the logic circuit 20 generates very small switching noise during its operation, but the analog-to-digital converter 10 is converting the analog signals into the digital signals even when the logic circuit 20 is operating so that the noise is transmitted to the power supply node NV and the ground node NG through power supply connections 30, 34 and ground connections 32, 36, respectively.
The capacitor 12 has its one plate connected to the ground node NG, so that the transmitted switching noise varies the voltage of the internal node NI via the capacitor 12 thereby worsening the accuracy of the analog-to-digital converter.
One approach to solving this type of problem is to apply the method described in Kobatake, e.g. to use different supply voltage and ground terminals for the analog-to-digital converter 10 and logic circuit 20, respectively, so that they are connected to different supply voltage and ground terminals. This approach, however, increases the number of supply voltage terminal and ground terminal and also the number of external connector pins of the LSI circuit whereby a pattern of the wiring board of the LSI circuit grows very complicated.